When ping6 is built with FEATURE_FANCY_PING set, echo requests sent to OS X and iOS based hosts do not generate replies consistently. The problem is the value of icmp6_code: RFC 4443 specifies it as 0, and both OS X and iOS actually check that, and only send an echo reply if it was in fact 0. (Other OSes are more permissive.) For some reason, BusyBox's sendping6 function (networking/ping.c, line ~400) has all the lines that would set a value of 0 for icmp6_code in its requests commented out. The code field and most of the rest of the packet is uninitialized memory (ie random data). The function also doesn't actually calculate or set the checksum value for the ICMPv6 header, but it's the non-zero code field that is causing the issue with Apple OSes. (Testing was done with BusyBox 1.18.2 on TomatoUSB, K26)
The fix appears to be: static void sendping6(int junk UNUSED_PARAM) { - struct icmp6_hdr *pkt = alloca(datalen + sizeof(struct icmp6_hdr) + 4); + struct icmp6_hdr *pkt = G.snd_packet; Can you confirm that it works?
Fixed in git and will go into 1.18.3